I use this type of cord often, for knot tying practice (a hobby of mine). Notice I said "type of cord". I have several different 5mm cords I practice with and they are all different. Some are stiffer, some are harder, some are softer, some are more flexible than others. They were all bought at outdoor recreation and/or climbing supply stores, so they are all good quality cord - just different from each other.

You can't really tell how a cord handles by looking at a picture or reading a description. And "holding a knot well" depends on the use. For example, stiff, hard cords do not hold bends as well as softer cords, but they're great when used for constrictor knots around softer material.

Thicker cords do not usually tend to jam up as much as smaller cords. So a smaller cord may feel like it holds "tighter" (because it's jammed). However, untying a knot in thinner cord can be near impossible sometimes. Sometimes you have to give up and cut the cord.

The above are just generalizations, and are not hard and fast rules for every cord in every situation. That's why you need to know several different knots for a given purpose. Depending on the specific cord you have, your previously tested "secure" knots may slip and other knots you considered less trustworthy may hold like a bulldog. In my experience, a softer more flexible cord tends to find use in more varied situations than a stiff or hard cord. YMMV of course.